A plane-mirror interferometer is described in U.S. Pat. No. 4,752,133 and includes a light source, a beam splitter, and a detector system. A beam of rays emitted by the light source is split via the beam splitter into at least one measuring beam and at least one reference beam. The measuring beam and the reference beam then propagate in a measuring arm and in a reference arm, respectively, until they are recombined at the beam splitter. In the measuring arm, a measuring reflector in the form of a plane mirror is provided on a movable object to be measured. The reference arm has at least one reference retroreflector. With the aid of the detector system, at least one distance signal is ascertainable from the recombined interfering measuring and reference beams with regard to the position of the object to be measured.
As described in U.S. Pat. No. 4,752,133, a polarization beam-splitter cube is provided as a beam splitter, and a triple prism is used as a reference retroreflector. Alternative arrangements of reference retroreflectors having a plurality of triple prisms may also be provided. The triple prism of the reference retroreflector is adhered onto or wrung to the beam-splitter cube. In addition, the triple prism may also be placed spatially separate from the beam-splitter cube.
It is disadvantageous with regard to such interferometers that both the polarization beam-splitter cube and the triple prism can only be produced with great expense. Thus, in the case of the beam-splitter cube, the various sides must be aligned highly exactly relative to each other, especially if one or more triple prisms and possibly further optical components are disposed directly on it. In the case of the spatially separate placement of beam-splitter cube and triple prisms, a very costly low-drift mounting of these components is likewise necessary. In addition, if great tilt tolerances of the object to be measured are demanded, the triple prism must be constructed to be relatively large.